This study proposes to set up an artificial capillary culture system for thymic nonlymphoid cells (TNLC), bone marrow adherent, bone fragment derived, spleen reticuloendothelial (BMAC, BFSC and SRE) cells fetal liver (hepatic) parenchymoendothelial (FHPE) cells and control fibroblasts. These cells will be grown in the extra-capillary space of artificial capillaries perfused with tissue culture medium and other culture systems. The medium obtained from TNLC cell or fibroblast cultures will be employed to condition cultures of T-cell deficient rat or mouse precursor cells (obtained from thymectomized irradiated bone marrow protected radiation chimeras and nude mice respectively) and these cells tested pre- and post-conditioning for helper and suppressor T cell characteristics employing a number of tests such as acquired antigens and mitogen responsiveness. The medium obtained from all these cultures, again using fibroblasts as a control, will be employed to condition hematopoietic stem cell concentrates prepared using nitrogen mustard pretreatment of donor mice followed by density gradient centrifugation. The stem cell concentrates will be tested pre- and post-conditioning for multipotent stem cell, progenitor cell, helper and suppressor T cell and B cell content. Also the proliferative fraction, erythroid fraction and differential count will be determined. The perfusate will be assayed for granulocyte-monocyte colony activation factor (CAF) and for other possible humoral effector substances, e.g., burst promoting activity. Hematopoietic stem cell concentrates will be circulated through these artificial capillary beds and characterized similarly. The objectives are to determine if the perfusate of the above stromal cell cultures has lymphocyte and/or hematopoietic stem cell proliferation and differentiation inducing properties, and further, if hematopoietic stem cell concentrates circulated through artificial capillaries show evidence of stimulation of proliferation and/or differentiation.